The main search engines and directories out there are often overwhelming for kids and present a lot content that is inappropriate. There are a number of sites that give you the ability to search for information that has been (to some degree or other) filtered for kids.
TekMom isn't fancy, but it is a one-stop shop with lots of useful sites and search engines, but not so many that it becomes overwhelming.
When kids are using Google or Yahoo!'s main sites, it's a good idea to make sure they have their preferences set to "Safe Search" or "Strict Safe Search". It won't block everything, but it's a good first line of defense, particularly when searching images. Again, it's tough to find on Yahoo! but there is a link right off of Google's home page for their preferences settings.
Another great place to start can be Answers.com. They collect content from authoritative resources, plus wikipedia, and present it in one page, complete with citations.
Good Sites for Kids
Directories
Reference & Research
Photos